Lithium accumulators are more and more often used to store power and thus provide an autonomous power source, in particular in portable equipment. They comprise a positive electrode and a negative electrode having an electrode separator ensuring the conduction of Li+ ions positioned therebetween.
Due to their more advantageous mass and volume energy densities, lithium accumulators tend to progressively replace nickel-cadmium (Ni—Cd) and nickel-metal hydride (Ni-MH) accumulators (the energy density is relative to the mass of the complete Li-ion cell).
Indeed, although the first Li-ion accumulators used to have an energy density of approximately 85 Wh/kg, the latest generations are close to 200 Wh/kg, that is, much more than the energy densities of Ni-MH accumulators (100 Wh/kg) and Ni—Cd accumulators (50 Wh/kg).
Thus, the next generations of lithium accumulators enable not only to widen the fields of use, but also stimulate the developing of new electrode materials to further increase their performance by improving the ratio of the energy to the mass and/or volume unit.
Generally, active positive electrode materials are layered compounds such as LiCoO2, LiNiO2 and mixed oxides of Li(Ni, Co, Mn, Al)O2 type, or compounds of spinel structure having a composition close to LiMn2O4.
Active negative electrode materials are generally based on carbon (graphite, coke, . . . ). They may however be of Li4Ti5O12 spinel type or a metal forming an alloy with lithium (Sn, Si, . . . ). The theoretical and practical specific capacities of such compounds are in the order of 370 mAh/g for graphite, and in the order of 175 mAh/g for titanium oxide.
Certain materials such as compound Li4Ti5O12 have a relatively high work potential, approximately 1.6 V, which makes them very safe. They can thus be used as a negative electrode material for power applications, particularly when they also have a very good high-rate cyclability, and this despite their low capacity as compared with that of graphite.
Since niobium has a work potential close to that of titanium, the search for alternatives to the Li4Ti5O12 compound as an electrode material has enabled to develop niobium oxides, but also mixed titanium-niobium oxides. It should further be noted that niobium enables to exchange 2 electrons per metal center (Nb5+/Nb3+).
Such prior art mixed Ti—Nb oxides are, in particular, the following:                ATiNbO5-type layered oxides, with A=H, Li, and the corresponding condensed phase Ti2Nb2O9 (Colin et al., “A Novel Layered Titanoniobate LiTiNbO5: Topotactic Synthesis and Electrochemistry versus Lithium”, Inorg. Chem., 45, 7217-7223, 2006; Colin et al., “Lithium Insertion in an Oriented Nanoporous Oxide with a Tunnel Structure: Ti2Nb2O9”, Chem. Mater., 20, 1534-1540, 2008; Colin et al., “New titanoniobates (Li,H)2TiNbO5 and (Li,H)3TiNbO5: synthesis, structure and properties”, J. Mater. Chem., 18, 3121-3128, 2008);        Wadsley-Roth-type phases, for example, TiNb2O7 and Ti2Nb10O29 (Cava et al., “Lithium Insertion in Wadsley-Roth Phases Based on Niobium Oxide”, J. Electrochem. Soc., 130, 2345-2351, 1983; Han et al., “New Anode Framework for Rechargeable Lithium Batteries”, Chem. Mater., 23, 2027-2029, 2011; Han et al., “3-V Full Cell Performance of Anode Framework TiNb2O7/Spinel LiNi0.5Mn1.5O4”, Chem. Mater., 23, 3404-3407, 2011).        
Compounds from the first group can generate first discharge capacities close to 250 mAh/g. However, they have problems of irreversibility.
Compounds from the second group such as TiNb2O7 and Ti2Nb10O29 appear to be particularly promising, particularly for applications requiring more energy, due their theoretical capacity (388 mAh/g and 396 mAh/g respectively), which is much greater than that of Li4Ti5O12 (175 mAh/g). They also enable to keep a work potential close to that Li4Ti0O12 and thus have the safety advantages thereof.
Document JP 2010-287496 describes the solid synthesis of the TiNb2O7 and Ti2Nb10O29 compounds from precursors TiO2 and Nb2O5. The two oxides thus prepared have respective capacities in the order of 270 mAh/g and 240 mAh/g.
Prior art describes the multiple-step sol-gel synthesis of TiNb2O7, by using hydrofluoric acid and citric acid. The low-rate performance of this compound reach 280 mAh/g but the capacities collapse during its high-rate use.
The high-rate cyclablity of the TiNb2O7 compound may be improved by coating with carbon and niobium doping. However, such techniques require specific atmosphere conditions (vacuum and argon), which are particularly difficult to implement at an industrial level.
Another alternative to the Li4Ti5O12 compound is the solid synthesis, in a plurality of steps, of the carbon-coated Ti0.9Nb21O7 material. It has a better high-rate behavior than TiNb2O7 obtained by sol-gel synthesis (190 mAh/g at 9C, that is, 67% of the low-rate capacity). However, such a synthesis requires using a controlled atmosphere for the doping and coating steps, respectively vacuum and argon, which is difficult to reproduce at an industrial scale.
Document EP 2 503 625 described a lithium material where a niobium portion is replaced with one or a plurality of elements. This results in a decrease in the melting temperature of the mixed titanium and niobium oxide, from 1,475° C. to 1,260° C.
The problem that the present invention aims at solving comprises providing a novel electrode material based on a mixed titanium and niobium oxide, having its specific capacity loss attenuated during charge/discharge cycles with respect to prior art mixed oxides.